P0155 fault code symptoms, causes, diagnosis, repair

What does the P0155 code mean?

The P0155 is a generic OBD-II powertrain trouble code. Its official definition is “O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 1)“.

This code is set when your vehicle’s Engine Control Module (ECM) detects a fault in the electrical heating circuit of the upstream oxygen sensor (Bank 2, Sensor 1). The heater is a critical component inside the O2 sensor. Its job is to quickly bring the sensor’s sensing element up to its optimal operating temperature (typically 600°F or more) after a cold start, allowing it to begin providing accurate exhaust gas readings faster. A faster warm-up reduces cold-start emissions and allows the engine to enter closed-loop fuel control sooner. The malfunction can be an open circuit (broken heater), a short circuit, or excessive resistance. The first step is to use a professional vehicle diagnostics tool to confirm the code and check for other related issues.

Symptoms of Error Code P0155

• Illuminated Check Engine Light (MIL).
• The vehicle may take longer to complete its emissions system self-tests, potentially causing it to fail an emissions inspection.
• Slightly decreased fuel economy, especially on short trips, as the engine may stay in “open loop” (using pre-programmed fuel maps) longer.
• In some cases, you may notice a slight increase in cold-start emissions or a brief rough idle until the sensor warms up from exhaust heat alone.
• Often, there are no noticeable drivability symptoms once the engine is fully warmed up.

Main Causes of Error Code P0155

• Faulty Oxygen Sensor Heater Element (Bank 2, Sensor 1): The most common cause. The internal heater coil inside the sensor has burned out or broken, creating an open circuit.
• Blown Fuse for the O2 Sensor Heater Circuit: A fuse dedicated to the oxygen sensor heaters (often shared between multiple sensors) may be blown.
• Damaged, Chafed, or Shorted Wiring: The wiring to the sensor’s heater circuit may be damaged, causing a short to ground, a short to power, or an open circuit.
• Corroded or Loose Electrical Connector: Corrosion, bent pins, or a poor connection at the sensor or harness connector can interrupt the heater circuit.
• Faulty Relay: Some vehicles use a relay to power the O2 sensor heaters. A failed relay can cut power to the circuit.
• Problem with the ECM Driver Circuit (Rare): An internal fault within the ECM that controls the heater circuit’s power.

How to Diagnose Error Code P0155?

1. Confirm Code and Check for Related Codes: Use a professional obd2 scanner to read and confirm P0155. Check if other O2 sensor heater codes (like P0135, P0141, P0161) are also present. Multiple codes point to a shared fuse or power supply issue.
2. Inspect the Fuse: Locate the fuse for the oxygen sensor heater circuit (consult the owner’s manual or a wiring diagram). Check it visually and with a multimeter for continuity. Replace if blown. Important: If the fuse blows again immediately, there is a short circuit that must be found and repaired.
3. Visual Inspection: Locate the Bank 2, Sensor 1 oxygen sensor. Inspect the wiring harness and connector for obvious damage, chafing, melting (from exhaust contact), or corrosion.
4. Test the Heater Circuit Resistance at the Sensor (Key Test):
   • Disconnect the electrical connector from the oxygen sensor.
   • Set a digital multimeter to measure resistance (Ohms, Ω).
   • Identify the two pins for the heater circuit (usually the two heavier-gauge wires of the same color; consult a wiring diagram).
   • Measure the resistance across these two pins on the sensor side of the connector. A typical reading is between 4 and 20 ohms when the sensor is cold. Compare to specifications if available.
     • Infinite resistance (OL): The heater is open (burned out) -> Sensor needs replacement.
     • Very low resistance (near 0 Ω): The heater is shorted -> Sensor needs replacement.
     • Resistance within spec: The heater element itself is likely okay; problem may be in the power supply or wiring.
5. Test for Power and Ground at the Vehicle Harness:
   • With the sensor still disconnected, turn the ignition to the ON position (engine off).
   • Using the multimeter set to DC Volts, check for battery voltage (approx. 12V) on the heater power supply wire in the vehicle’s harness connector.
   • If no voltage is present, the problem is in the power supply circuit (fuse, relay, wiring back to the ECM).
   • Also, verify a good ground on the heater ground wire in the harness.
6. Check for Continuity in the Wiring: If the sensor tests bad or if no power is found, you may need to check for continuity and shorts in the wiring between the sensor connector and the fuse box/ECM.

Possible Causes and Diagnostic Methods

Tools needed: A professional car scanner to read codes, a digital multimeter, basic hand tools, and a vehicle-specific wiring diagram for pin identification.

How do I fix error code P0155? (Solutions to the Problem)

Simple Fixes

• Replace a Blown Fuse: If the fuse is blown, replace it with one of the correct amperage. If it blows again, stop and find the short circuit.
• Clean and Reconnect Electrical Connectors: Disconnect the battery. Clean the oxygen sensor and vehicle harness connectors with electrical contact cleaner. Ensure a tight, corrosion-free connection.

In-depth Diagnosis and Repair Solutions

• Replace the Oxygen Sensor (Bank 2, Sensor 1): This is the most common repair if the heater resistance is out of specification (open or shorted). Replace the entire sensor unit. The heater is not a serviceable part separate from the sensor.
• Repair Damaged Wiring: If the wiring is chafed, cut, or shorted, repair it properly with solder and heat-shrink tubing. Ensure the harness is re-secured away from heat and moving parts.
• Replace a Faulty Relay: If diagnostic steps point to a bad power relay for the heater circuit, replace the relay.
• Address an ECM Fault (Professional Required): If all wiring and components test good, the issue may be within the ECM. This requires professional diagnosis and potentially ECM replacement or reprogramming.

Fix faults based on symptoms

Common Error Code P0155 in Vehicles

P0155 is a very common code across all manufacturers, as the heater element is a wear item that eventually fails due to thermal cycling. It is especially prevalent in vehicles with over 80,000 miles.

Ford Vehicles (All Models): Extremely common in Ford F-150, Explorer, Focus, Fusion, and Mustang. The upstream O2 sensors are known for heater failures. A vehicle code reader will confirm the code, and the fuse for the heaters is often located in the battery junction box under the hood.

General Motors (Chevrolet, GMC, Buick, Cadillac): Very common in Silverado, Sierra, Malibu, Equinox, and many others. GM vehicles often use a single fuse (e.g., a 20A fuse labeled “O2 Sensor” or “ECM”) to power all four sensor heaters. If this fuse blows, it will set multiple heater codes at once.

Toyota and Lexus: Common in Camry, Corolla, RAV4, Highlander, and Lexus ES/RX models. Toyota’s sensors are durable, but the heaters do fail with age. Using a car scanner to check for this specific code is the first indication.

Honda and Acura: Frequent in Civic, Accord, CR-V, Odyssey, and Acura TL, MDX. The sensors are located in a hot environment, and the heater circuits are prone to failure.

Chrysler, Dodge, Jeep, Ram: Common in Caravan, Charger, Cherokee, and Ram trucks. The heater circuit is simple and failure usually points directly to the sensor.

Nissan and Infiniti: Found in Altima, Sentra, Rogue, Pathfinder, and Infiniti G/QX models. The upstream sensors are a standard replacement item.

European Vehicles (Volkswagen, Audi, BMW, Mercedes-Benz): These vehicles are also susceptible. In VW/Audi, a P0155 points to a failing lambda sensor heater on Bank 2. Diagnosis is similar but may require a more advanced vehicle diagnostics tool to view readiness codes after repair.

Hyundai and Kia: Common in Sonata, Elantra, Santa Fe, Sorento, and Sportage. As high-mileage maintenance, sensor replacement is typical.

Subaru: Appears in Outback, Forester, Impreza, and Legacy models, especially in colder climates where the heater is used more intensely.

P0155 Frequently Asked Questions

Is it safe to drive with a P0155 code?

Yes, it is generally safe to drive. The engine will still run, and the oxygen sensor will eventually warm up from exhaust heat. However, you will have increased emissions during warm-up, potentially fail an emissions test, and may see slightly reduced fuel economy. The problem will not cause immediate drivability issues, but it should be repaired to restore optimal engine performance and emissions compliance.

What’s the difference between P0150 and P0155?

P0150 indicates a general circuit malfunction in the oxygen sensor’s signal circuit. P0155 is specifically for a malfunction in the sensor’s built-in heater circuit. They are two separate circuits within the same sensor.

Can a P0155 code cause other problems?

Primarily, it causes increased emissions. Over a very long time, a constantly cold sensor might provide slightly less accurate readings during warm-up, but it is unlikely to cause major engine damage. The main risk is failing an emissions test and increased pollution.

How much does it cost to fix P0155?

Costs are straightforward: Diagnosis: $80-$100. A new upstream oxygen sensor: $60-$250. Labor: 0.5-1 hour ($50-$100). Total cost typically ranges from $150 to $400, depending on the vehicle and sensor cost. DIY repair (sensor and basic tools) can cost as little as $60-$250 for the part.

Do I need a special scan tool to diagnose P0155?

A basic car code scanner can confirm the P0155 code exists. However, to properly diagnose it, you will need a digital multimeter to test the heater resistance and circuit voltage. A professional vehicle diagnostics tool is helpful for checking related codes and clearing the code after repair.

Will clearing the code with a scanner fix P0155?

No. Clearing the code with an obd2 scanner will turn off the Check Engine Light temporarily. However, if the faulty heater, blown fuse, or wiring problem is still present, the code will return as soon as the ECM runs its heater circuit monitor again, usually within one or two drive cycles.

Can I drive with a bad O2 sensor heater?

Yes, as explained above. The vehicle is driveable, but you should plan to fix it to avoid emissions test failure and to restore optimal engine operation.

How long does an oxygen sensor last?

The sensor itself can last 60,000 to 100,000 miles or more. The heater element, due to constant heating and cooling cycles, is often the first part to fail.

Can I replace just the heater?

No. The heater is an integral, non-serviceable part of the oxygen sensor assembly. If the heater circuit fails, the entire oxygen sensor must be replaced.

Why did my code come on in cold weather?

The heater works hardest in cold weather to bring the sensor up to temperature. A weak or failing heater is more likely to be detected by the ECM’s monitor during cold starts in winter, triggering the code.

P0155 Related OBD2 Errors

• P0135: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
• P0141: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
• P0161: O2 Sensor Heater Circuit Malfunction (Bank 2, Sensor 2)
• P0150: O2 Sensor Circuit Malfunction (Bank 2, Sensor 1)
• P0151: O2 Sensor Circuit Low Voltage (Bank 2, Sensor 1)
• P0152: O2 Sensor Circuit High Voltage (Bank 2, Sensor 1)

Important! Code P0155 is specifically for the heater circuit, not the sensing circuit. Always test the heater resistance with a multimeter before replacing the oxygen sensor. Also, always check the corresponding fuse first, as it is a simple and inexpensive fix. Ignoring this code will cause your vehicle to fail an emissions test, and over time, may lead to slightly reduced fuel efficiency.